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<tr class="t-nv-h2"><td colspan="5"> 工具组件</td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/swap.html" title="cpp/experimental/ranges/utility/swap"><span class="t-lines"><span>swap</span></span></a></div></div></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> 函数对象</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/invoke.html" title="cpp/experimental/ranges/functional/invoke"><span class="t-lines"><span>invoke</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/identity.html" title="cpp/experimental/ranges/functional/identity"><span class="t-lines"><span>identity</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/equal_to.html" title="cpp/experimental/ranges/functional/equal to"><span class="t-lines"><span>equal_to</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/not_equal_to.html" title="cpp/experimental/ranges/functional/not equal to"><span class="t-lines"><span>not_equal_to</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/greater.html" title="cpp/experimental/ranges/functional/greater"><span class="t-lines"><span>greater</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/less.html" title="cpp/experimental/ranges/functional/less"><span class="t-lines"><span>less</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/greater_equal.html" title="cpp/experimental/ranges/functional/greater equal"><span class="t-lines"><span>greater_equal</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../functional/less_equal.html" title="cpp/experimental/ranges/functional/less equal"><span class="t-lines"><span>less_equal</span></span></a></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 元编程与类型特性</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="is_swappable.html" title="cpp/experimental/ranges/type traits/is swappable"><span class="t-lines"><span>is_swappable_with</span><span>is_swappable</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="is_swappable.html" title="cpp/experimental/ranges/type traits/is swappable"><span class="t-lines"><span>is_nothrow_swappable_with</span><span>is_nothrow_swappable</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>common_reference</span></span></strong></div></div></td></tr>
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</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 有标签的 pair 与 tuple</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/TagSpecifier.html" title="cpp/experimental/ranges/utility/TagSpecifier"><span class="t-lines"><span>TagSpecifier</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/TaggedType.html" title="cpp/experimental/ranges/utility/TaggedType"><span class="t-lines"><span>TaggedType</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5">                          </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/tagged.html" title="cpp/experimental/ranges/utility/tagged"><span class="t-lines"><span>tagged</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm/tags.html" title="cpp/experimental/ranges/algorithm/tags">标签说明符</a></td></tr>
<tr class="t-nv"><td colspan="5">                                      </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/tagged_pair.html" title="cpp/experimental/ranges/utility/tagged pair"><span class="t-lines"><span>tagged_pair</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/make_tagged_pair.html" title="cpp/experimental/ranges/utility/make tagged pair"><span class="t-lines"><span>make_tagged_pair</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5">                          </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/tagged_tuple.html" title="cpp/experimental/ranges/utility/tagged tuple"><span class="t-lines"><span>tagged_tuple</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../utility/make_tagged_tuple.html" title="cpp/experimental/ranges/utility/make tagged tuple"><span class="t-lines"><span>make_tagged_tuple</span></span></a></div></div></td></tr>
</table></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../../../header/experimental/ranges/type_traits.html" title="cpp/header/experimental/ranges/type traits">&lt;experimental/ranges/type_traits&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-ranges">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span>... <span class="me1">T</span> <span class="sy1">&gt;</span><br>
<span class="kw1">struct</span> common_reference<span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-ranges-ts t-mark-ts">(范围 TS)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>确定类型 <code>T...</code> 的公共引用类型，即 <code>T...</code> 中所有类型能转换或绑定到的类型。若这种类型存在（如按照后述规则所确定），则成员 <code>type</code> 指名该类型。否则，无成员 <code>type</code> 。若 <code>T...</code> 中的任何类型为（可有 cv 限定的） <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 以外的不完整类型，则行为未定义。
</p><p>给定引用类型是， <code>common_reference</code> 试图寻找所有提供的引用类型都能绑定到的引用类型，但若找不到这种引用类型，则可能返回非引用类型。
</p>
<ul><li> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>T<span class="br0">)</span></span></span> 为零，则无成员 <code>type</code> 。
</li><li> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>T<span class="br0">)</span></span></span> 为一（即 <code>T...</code> 仅含一个类型 <code>T0</code> ），则成员 <code>type</code> 指名与 <span class="t-c"><span class="mw-geshi cpp source-cpp">T0</span></span> 相同的类型。
</li><li> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>T<span class="br0">)</span></span></span> 为二（即 <code>T...</code> 含二个类型 <code>T1</code> 和 <code>T2</code> ），则：
<ul><li> 若 <code>T1</code> 和 <code>T2</code> 都是引用类型，而 <code>T1</code> 和 <code>T2</code> 的<i>简单共用引用类型</i> <code>S</code> 存在，则成员类型 <code>type</code> 指名 <code>S</code> ；
</li><li> 否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">basic_common_reference<span class="sy1">&lt;</span>T1R, T2R, T1Q, T2Q<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span></span></span> 存在，其中 <code>TiR</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1">&lt;</span><a href="../../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>Ti<span class="sy1">&gt;&gt;</span></span></span> ，而 <code>TiQ</code> 为使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">TiQ<span class="sy1">&lt;</span>TiR<span class="sy1">&gt;</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ti</span></span> 的别名模版，则成员类型 <code>type</code> 指名该类型；
</li><li> 否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw2">false</span><span class="sy4">?</span> val<span class="sy1">&lt;</span>T1<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> val<span class="sy1">&lt;</span>T2<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 是合法类型，其中 <code>val</code> 为函数模板 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> T val<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> ，则成员类型 <code>type</code> 指名该类型；
</li><li> 否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">common_type_t</span><span class="sy1">&lt;</span>T1, T2<span class="sy1">&gt;</span></span></span> 为合法类型，则成员类型 <code>type</code> 为该类型；
</li><li> 否则，无成员 <code>type</code> 。
</li></ul>
</li><li> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>T<span class="br0">)</span></span></span> 大于二（即 <code>T...</code> 由 <code>T1, T2, R...</code> 组成），则若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1">&lt;</span>T1, T2<span class="sy1">&gt;</span></span></span> 存在，且 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1">&lt;</span>ranges<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1">&lt;</span>T1, T2<span class="sy1">&gt;</span>, R...<span class="sy1">&gt;</span></span></span> 存在，则成员类型 <code>type</code> 指代它。所有其他情况下，无成员类型 <code>type</code> 。
</li></ul>
<p>二个引用类型 <code>T1</code> 和 <code>T2</code> 的<i>简单共用引用类型</i>定义如下：
</p>
<ul><li> 若 <code>T1</code> 为 <code><i>cv1</i> X &amp;</code> 而 <code>T2</code> 为 <code><i>cv2</i> Y &amp;</code> （即都是左值引用）：则其简单共用引用类型为 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw2">false</span><span class="sy4">?</span> <a href="http://zh.cppreference.com/w/cpp/utility/declval"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1">&lt;</span>cv12 X <span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> <a href="http://zh.cppreference.com/w/cpp/utility/declval"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1">&lt;</span>cv12 Y <span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> ，其中 <i>cv12</i> 为 <i>cv1</i> 与 <i>cv2</i> 的联合，若该类型存在且为引用类型；
</li><li> 若 <code>T1</code> a与 <code>T2</code> 均为右值引用类型：若 <code>T1 &amp;</code> 和 <code>T2 &amp;</code> 的简单共用引用类型（按照前一条确定）存在，则令 <code>C</code> 代表该类型的对应右值引用类型。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/is_convertible.html"><span class="kw592">std::<span class="me2">is_convertible</span></span></a><span class="sy1">&lt;</span>T1, C<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/is_convertible.html"><span class="kw592">std::<span class="me2">is_convertible</span></span></a><span class="sy1">&lt;</span>T2, C<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span></span></span> 均为 <code>true</code> ，则 <code>T1</code> 与 <code>T2</code> 的简单共用引用类型为 <code>C</code> 。
</li><li> 否则，二个类型之一必须为左值引用类型 <code>A &amp;</code> 而另一个必须为右值引用类型 <code>B &amp;&amp;</code> （ <code>A</code> 与 <code>B</code> 可为 cv 限定）。令 <code>D</code> 代表 <span class="t-c"><span class="mw-geshi cpp source-cpp">A <span class="sy3">&amp;</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">B <span class="kw4">const</span> <span class="sy3">&amp;</span></span></span> 的简单共用引用类型，若它存在。若 <span class="t-c"><span class="mw-geshi cpp source-cpp">D</span></span> 存在且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/is_convertible.html"><span class="kw592">std::<span class="me2">is_convertible</span></span></a><span class="sy1">&lt;</span>B <span class="sy3">&amp;&amp;</span>, D<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span></span></span> 为 <code>true</code> ，则简单共用引用类型为 <code>D</code> 。
</li><li> 否则，无简单共用引用类型。
</li></ul>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">  
</td></tr>
<tr class="t-dsc-hitem">
<td> 名称
</td>
<td> 定义
</td></tr>


<tr class="t-dsc">
<td>  <code>type</code>
</td>
<td> 所有 <code>T...</code> 的共用引用类型
</td></tr>
</table>
<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E7.B1.BB.E5.9E.8B">辅助类型</span></h3>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span>... <span class="me1">T</span> <span class="sy1">&gt;</span><br>
<span class="kw1">using</span> common_reference_t <span class="sy1">=</span> <span class="kw1">typename</span> common_reference<span class="sy1">&lt;</span>T...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T, <span class="kw1">class</span> U, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span><span class="sy1">&gt;</span> <span class="kw1">class</span> TQual, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span><span class="sy1">&gt;</span> <span class="kw1">class</span> UQual <span class="sy1">&gt;</span><br>
<span class="kw1">struct</span> basic_common_reference <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>类模板 <code>basic_common_reference</code> 是定制点，允许用户影响 <code>common_reference</code> 对用户定义类型（常为代理引用）的结果。初等模板为空。
</p>
<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96">特化</span></h3>
<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1">&lt;</span>T, <a href="../../../types/decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;&gt;</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1">&lt;</span>U, <a href="../../../types/decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;&gt;</span></span></span> 皆为 true ，且它们至少有一个依赖于用户定义类型，则程序可以在前二个形参 <code>T</code> 和 <code>U</code> 上特化 <code>basic_common_reference&lt;T, U, TQual, UQual&gt;</code> 。
</p><p>若这种特化拥有名为 <code>type</code> 的成员，则它必须是指名 <span class="t-c"><span class="mw-geshi cpp source-cpp">TQual<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">UQual<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span></span></span> 都能转换到的类型的公开且无歧义的成员类型。另外， <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">basic_common_reference</span><span class="sy1">&lt;</span>T, U, TQual, UQual<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">basic_common_reference</span><span class="sy1">&lt;</span>U, T, UQual, TQual<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span></span></span> 必须指代同一类型。
</p><p>程序不可以在第三或第四形参上特化 <code>basic_common_reference</code> ，亦不可特化 <code>common_reference</code> 自身。以违背这些规则的方式添加特化的程序拥有未定义行为。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">本节未完成 </td></tr></table>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">本节未完成<br>原因：暂无示例 </td></tr></table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../../../types/common_type.html" title="cpp/types/common type"> <span class="t-lines"><span>common_type</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>   确定一组类型的公共类型  <br> <span class="t-mark">(类模板)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="common_type.html" title="cpp/experimental/ranges/type traits/common type"> <span class="t-lines"><span>common_type</span></span></a></div></div>
</td>
<td>  确定类型集合的公共类型 <br> <span class="t-mark">(类模板)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../concepts/CommonReference.html" title="cpp/experimental/ranges/concepts/CommonReference"> <span class="t-lines"><span>CommonReference</span></span></a></div></div>
</td>
<td>  指定两个类型共有一个公共引用类型  <br> <span class="t-mark">(概念)</span> </td></tr>
</table>

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